1. Field of the Invention
This invention relates generally to methods and apparatus for dosing liquids, and more particularly to a method and apparatus for increasing the dosing precision of a dosing pump.
2. Discussion of Related Art
Dosing pumps are used for the precision dosing of different liquids. Typically a dosing pump is actuated by external pulses, by a standard signal or by an internal clock. The external pulses can also be generated, for example, by a liquid counter and optionally multiplied or divided. Through use of an eccentric gear, the diaphragm or the piston of the pump is supposed to complete a stroke, the maximum stroke frequency generally amounting to about 125 strokes/minute.
Asynchronous motors are preferably used for dosing for metering pumps with a motor output of about 40 watts or higher. Through their moment of inertia, asynchronous motors are capable of slowing down without braking when switched off. This results in overdosing. With motor outputs of about 40 to 60 watts, the slowing down responsible for this overdosing can be reduced, but not avoided, by stiffening the mounting or by fitting a pressure-retaining valve. A particular problem can arise where the diaphragm is driven through an eccentric gear. If the motor comes to a stop outside its dead center, the starting torque for the next cycle can exceed the permitted value so that the motor does not start.
Mechanical brakes are unsuitable for stroke frequencies of more than 3- strokes/minutes. Hitherto, electrical brakes have only been practicable for motors with an output of more than 1 kilowatt and for braking times of more than 1 second. In the case of electromagnetic braking, the electromagnetic forces are in any case only used to drive mechanical brakes. With eddy-current brakes, additional external parts are necessary which is expensive and makes such brakes impossible to use for small motors.